Electron beam device and system employing space charge neutralization



D 1952 E. e. I ER ELECTRON BEAM DEVICE SYSTEM EMPLOYING SPACE CHARGENEUTRALIZATION Filed D60. 31, 1948 INVENTQR may; fiIZ Fr 1'- {W ATTORNEYPatented Dec. 16, 1952 UNITED S'E'T ELECTRON BEAM DEVICE AND SYSTEMEMPLOYING SPACE CHARGE NEU- TRALIZATION Ernest G. Linder, Princeton, N.J., assigncr to Radio Corporation of America, a corporaticn ofDelawareThis invention relates generally to high current electron guns and moreparticularly to such guns incorporated in microwave apparatus, such asoscillators and modulators.

In applying electron guns to microwave devices and their associatedcircuits, it has been most desirable in the past that the electron beambe fine, dense and concentrated. These qualities have been somewhatimproved by the use of various electron focussing devices particularforms of field-free arrangements.

However, while arrangements have been proposed for shielding the beamfrom extraneous fields, for example, by enclosing it in a metalenvelope, the only solutions which have been devised for eliminating thespace charge field of the beam itself have required that the beam besurrounded by a gaseous medium at a critical and substantial pressure.Nor has the prior art taught utilizing concentration and dispersion ofthe beam as a step in the method of using th phenomenon of ion trappingfor new and useful purposes of generating, modulating, amplifying, andmixing of currents.

The principal object of the invention is to provide a method of andapparatus for controlling the density of a high current density electronbeam.

Another object of the invention is to provide a method of and apparatusfor concentrating a stream of electrons into a fine and dense beam.

Another object of the invention is to provide a method of and apparatusfor concentrating an electron beam that it maypass through restrictedapertures.

Another object of the invention is to provide a method of and apparatusfor concentrating and dispersing an electron beam as it approaches arestricted aperture.

Another object of the invention is to trap posi. tive ions, formed bythe collision of the electrons in a beam with the residual molecules ofthe mediurn'through which the beam passes, by establishing electricfields at boundaries within the medium.

Another object of the invention is to control the density of an electronbeam asit approaches an aperture to control the portion of electrons inthe beam that pass through the aperture and thereby accomplish currentmodulation.

Other objects will be apparent from the description of the invention ashereinafter set forth in detail and from the drawings made a part h eqie'whicb ur diagrammatic sketch of cne embodiment of the invention; Fin

are 21 is a diagrammatic sketch showing the optimum concentrating effecton a focussed electron beam of a space within the beam that is free ofspace charge; Fig. 2b is a similar sketch showing the efiect of themagnetic field 'on a convergent electron beam; Figure 3 is a"diagrammatic sketch of the invention applied to a refiex- Klystron;Figure 4 is a modification of the arrangement of Figure 3 showing aparamagnetic ring within the Klystron tube to concentrate the magneticfield Within a definite region of the tube; and Figure 5'is adiagrammatic sketch of the external circuits applied to Figure 2ato'accomplish modulation.

Similar reference characters are applied to similar elements throughoutthe drawings.

Figure 1 illustrates an electron gun in which the numeral l indicates acathode, shown "as spherical in shape, which is the source of a focussedelectron beam. Paths of individual electrons are shown at 2. Y Theelectrons pass through the region shown generally at 3, through anapt-2rture 4 in a cup-shaped electrode id to one side of electrode 5,where they are repelled or collected according to the use made of thegun. The side walls 4b of electrode 6a extend back to 'anelectron-permeable grid 8. Theelectrode la and grid 8 form a hollowconductive member that is substantially closed on all sides and servesas a Faraday cage to shield the interior'thereof from external electricfields.

Juxtaposed to the other side of electrode 5 is a source 6 of a magneticfield which permeates the region 3. Themagnetic fiux lines are ingeneral parallel to the paths o f the electrons in the beam, asindicated by the dotted lines 1. The dimensions and shape of themagnetic field sourcemay be chosen so that the flux field may have theproper configuration. Electrode 8, in the form of a grid, is positionednear cathode i and conforms generally in shape to spherical cathod' lElectrode '8 and cathode l have a common center of curvature, which islocated at, or near, aperture t. The electrodes 1, 4a, 5 and 8 arelocated'within an envelope c of magnetic field permeable materialcontaining a gaseous medium. I

Unless grid 8 is used for modulation or amplification, its potentialismaintained the same as that of electrode id and under such conditionselectrode 8 maybe connected to the sidewalls ib of electrode 4a as showninFig. l.

Referring to Figure 2a, the Q QQF Z PS emit from spherical cathode" ltend to follow"the straight radial paths 2 and focus at the apertureinelectrode la'a'nd would do so excep't'for repulsive effects within thespace charge of the electrons within the beam and the electrons producedby ionization of molecules in the gaseous medium by the electrons in thebeam.

However, in operation, the space charge eflects cannot be disregarded asthey affect to a great extent the degree of concentration of the beamwhich in turn affects the use that can be made of the beam. When theinvention is applied to an oscillator, maximum accuracy in theconcentration of the beam at aperture 4 i desired so that the number ofelectrons which can pass therethrough and therefore the output energy ofthe oscillator will be as high as possible;

When the invention is applied to a modulator, maximum variation in theaccuracy of said concentration is desired to obtain maximum variationsin the output electron current passing through the aperture 5.

The space charge effects are overcome by placing substantially equalpotentials on electrodes 4a and 8.

Electrodes 4a and 8 provide a field-free space therein, except for thefield due to the electron charge in the beam. This beam field performstwo functions: (1) it attracts and traps into the beam ions formed byionizing collisions and (2) it repels the electrons created by the saidcollisions as they are for the most part of low velocity. This processcontinues until the ions present in the beam neutralize the space chargefield of the beam. A trapping equilibrium is thereby established. It hasbeen found in practice that gas pressure of the medium is not criticaland may be as low as that of the residual gas remaining afterevacuation. Gas pressures affect the time of the device to reach a stateof equilibrium, but even with wide range of gas pressures the variationsin build-up time is only a matter of a very few microseconds. The beamspace thereby becomes actually field-free. The electrons in the beamalso respond to the flux of the magnetic field impressed upon region 3.The path of least resistance for an electron in a magnetic field is thatpath that does not cross the magnetic lines of force. The electrons,therefore, tend to follow the flux paths and the magnetic field tends tokeep the beam of electrons from spreading.

The effects of the magnetic field and the electric field set up by thepotentials on electrodes 4a and 8 cooperate with each other to produceimproved results. The optimum effect obtainable with the electric fieldalone is shown in Figure 2a in which figure it Will be seen that theangle of convergence of the electrons, as they approach the aperture 4in electrode 4a, is wide for short spacing of cathode I from electrode4a. On the other hand as shown in Fig. 2b, the magnetic flux lines I aresubstantially perpendicular to the cathode surface in the region nearthe cathode I and become substantially parallel and closer to each otherin the region of aperture 4. Due to the influence of the magnetic fieldthe paths of the electrons passing through and beyond the aperture arepractically parallel and are closer to each other, with attendingincrease in density of the beam, permitting the use of smallerapertures. The magnetic field prevents the cross-over of electrons nearthe aperture 4 shown in Fig. 2a.

Figure 3 discloses the application of the invention to a reflex-Kylstronoscillator, The device includes a conductive envelope 9 enclosing agaseous medium. A reflector is supported by a .4 member ID made of amagnetic-field permeable insulating material, such as a suitableceramic. Lugs H extend away from envelope 9' and in these lugs arethreaded screws I2. These screws [2 hold magnet core 6 in position and,by the adjustment of screws 12, magnetic core 6 may be centered in theaxis of envelope 9.

The cathode I is heated by electric currents which may be supplied tocathode I over wires M by connecting any suitable source to theterminals 13. Positive potentials are applied to the cavity resonator I5and envelope 9, which constitute the equivalent of electrode 4a ofFigure l, and to grid 8 through wires l6 and ll, respectively. Reflectornegative potential is applied to electrode 5 over a lead l8.

The electron beam passes through the apertures of cavity resonator I5into the drift space between cavity resonator l5 and reflector electrode5 where the electrons are bunched. The bunched electrons are reflectedback into cavity resonator I5. Power is drawn from cavity resonator 15by loop I9 and coaxial transmission line 20.

The device disclosed in Figure 4 is similar to that in Figure 3, exceptthat a paramagnetic ring 21 is positioned within envelope 9' to shapethe electromagnetic field from core 6 into the desired configuration andthus effect a longer narrow section of the electron beam.

In the devices shown in Figures 3 and 4, a substantially field-freeregion for trapping positive ions is formed by the conductive envelope9. the cavity resonator l5 and the grid 8.

In testing the devices herein disclosed and in obtaining theiroperational characteristic curves, various values of potentials wereapplied to the grid 8 and electrode 4a with various values of magneticfields, and Without any magnetic field impressed on region 3. It wasfound for the devices tested that the positive potential on grid 8should be substantially equal to that on electrode 441 to provide afield-free region therebetween. By impressing a potential on grid 8higher than that on electrode 4a, the positive ions will move towardelectrode 4a. This direction of movement of the positive ions isgenerally preferred as the concentration of the positive ions near theaperture 4 assures more effective trapping of the ions in the beam as itapproaches electrode 40. and thus a finer and more concentrated beam asit passes through aperture 4, which is desired.

It was also found that when the positive potential on grid 8 was loweredwith respect to electrode 4a, the positive ions were withdrawn or sweptfrom region 3 and the beam was dispersed. The electrons passing throughaperture 4 were thereby decreased, causing a decrease in the value ofthe oscillating current.

The variation of potential on electrode 8, due to superimposing onelectrode 8 the potentials set up by the modulating current, causes somevariations of the acceleration of the electrons and the density of theelectrons in the beam. These latter variations are minor compared withthe variations in the size and density of the beam caused by the effectof the existence or the neutralization of the excess space charge, whichis controlled by the varying potential on electrode 8.

The device may therefore be used as a modulator by impressing upon grid8 a varying potential from a modulating current source, such as shown at22, Figure 3. A constant positive potential is placed on grid 8 byelectric source 23, which is connected between source 22 and the groundorianeutralpotential point, such as one oiithei'wires; lllthatzsuppliescurrentt cathode; l-. Reierringto Figure 5 there is shown thereintheconstruction arrangements of Figure 2a and the'circuit arrangementsto modulate the currentin a: steady electron focussed beam originatingatcathode I and to collect the electrons at electrode 5 as. amodulatedcurrent. Electrodes 4a and, 85 are maintained at substantially the sameaverage positive potentials by electric sources 24 and; 23;.respectively. Superimposed upon; the

potentialof electrode 5 due to source 23; is a potential varying inaccordance with characteristics of themodulating current. This isaccomplishedby connecting source 22 to the primary of transformer 25,the secondary of which is connected; in series between electrode 8 andsource 23: The, load or output circuit of the device consists ofcollector electrode 5, load re-.

sistance 25', electric source 2'? and ground 28. Electric sources 23 and24 are grounded at 28.

The operation of the device is as previously described. The varyingpotential on electrode 8 varies the extent to which the region 3' iscompletely fieldefree, i. e., varies the field configuration of theion-trapping substantially field-free space and, therefore, the accuracywith which the. beam is brought to a sharp focus at the aperture 4.Obviously this controls the portion of electrons of the beam that passthrough aperture 4, to be collected by electrode 5. The varying number.of electrons collected by electrode 5 constitutes the modulated currentwhich is utilized. by passing it through load 25.

In general, the cathode, may be positioned much fart-her away from thecavity resonator l5 than in the conventional refiex-Klys ron tube as thespace charge limitations in such tubes do not occur in the case of thestructures described herein. Because of this, the time of transit ofelectrons returning toward the cathode is longer, practically completedebunching occurs, and harmful hysteresis due to multiple transits areeliminated.

There is thus disclosed a method of and an apparatus for controlling thedensity of an electron beam as it approaches an aperture to concentrateit by neutralization of the excess space charge for the use of the beamin an oscillator structure or to concentrate or disperse the beam inaccordance with the characteristics of a varying current to modulate thebeam and thereby produce a modulated current.

I claim as my invention:

1. Apparatus for producing a high current density electron beamcomprising a hollow conducting member substantially closed on all sidesto shield the interior thereof from external electric fields, at least apart of one wall of said member being electron permeable, the oppositewall of said member having a small beam aperture, a gaseous mediumwithin said member, and means including a cathode of large area relativeto said aperture located adjacent to said member for projecting aconvergent focussed beam of electrons through said electron-permeableWall part and said beam aperture to produce positive ions in said mediumwhich are trapped within said convergent beam for neutralizing the spacecharge thereof.

2. Apparatus according to claim 1, wherein said cathode and saidelectron permeable wall part lie in two spherical surfaces having acommon center on the axis of said convergent beam.

3. Apparatus according to claim 1, wherein said le tronerm able, W l: ars ctri ally sulatedas to; directcur-rents; from the remainden of; said;hollow conducting member, and saidapparatus includes means for applyinga variable voltage tosaid wall part.

i. Apparatus for producing oscillationsirr a medium including means forprojecting; electronsalong such paths that they tenduto produce afocussed electron beam, means providing a substantially field-freeregion through which said paths extend, a cavity resonator including.opposing-conductive Walls each having :an opening which surrounds theaxis of said beamand-is, spaced from the other to define. an interactiongap, a. gaseous medium within said region, and electron reflector meanspositioned in said axis and on the side of said cavity resonatoroppofiteto said electron projecting means, whereby oscillationsiare producedinsaid cavity resonator and whereby positive ions formed by theionization of the medium are trapped by said electrons in said regionand neutralize the space charge therein.

5. Apparatus according to claim 4, including electrode means, forming apart of said secondnamed means, for varying the field configuration ofsaid substantially field-free region.

6. Apparatus for producing a high-currentdensity electron beamcomprising: a source supplying a large diffuse electron current, means,adjacent said source for accelerating the elece trons away therefromalong paths which tend to converge at a given point to form anapex,means, between said source and said point for providing a substantiallyfield-free region therebetween, and a gaseous medium within said regionwhereby in the operation of the apparatus positive ions. of'said mediumare entrapped by electrons within said region to neutralize theirnegative space charge, and meansv adjacent said. region for impressingupon said beam a magnetic field the flux lines of which aresubstantially parallel to said paths of the electrons.

7. Apparatus for producing a variable. density electron beam comprising:a source supplying a large diffuse electron current, means adjacent saidsource for accelerating the electrons away therefrom along paths whichtend to be convergent at a given point to form an apex, an electrodehaving a beam-limiting aperture at said point, means between saidsource. and said point for providing a substantially field-free regiontherebetween, a gaseous medium in said region whereby in the operationof the apparatus positive ions of said medium are entrapped by electronswithin said region to substantially neutralize their negative spacecharge, an electrode partially bounding said region for varying thefield configuration of said substantially field-free region whereby saidentrapment and said neutralization will vary in accordance with thevariations of the field configurations and the electron current whichpasses through said aperture will also vary in accordance therewith, andmeans adjacent said region for impressing upon said beam a magneticfield the flux lines of which are substantially parallel to said pathsof the electrons.

8. Apparatus according to claim 1, wherein said cathode includes anelectron-emissive surface which is concave toward said aperture, andsaid apparatus further comprises means adjacent said hollow conductingmember for impressing upon said beam a magnetic field the flux lines ofwhich are substantially perpendicular to said cathode surface in theregion near said cathode and become substantially parallel and closer toeach other in the region near said aperture, whereby cross-over ofelectrons near said aperture is prevented.

9. Apparatus for producing oscillations in a medium including: means forprojecting electrons along such paths that they tend to produce afocussed electron beam, means providing a substantially field-freeregion through which said paths extend, a cavity resonator includingopposing conductive walls each having an opening which surrounds theaxis of said beam and is spaced from the other to define an interactiongap, said last-mentioned means including a gaseous medium within saidregion, electron reflector means positioned in said axis and on the sideof said cavity resonator opposite to said electron projecting means,whereb; oscillations are produced in said cavity resonator and wherebythe positive ions formed by the ionization of the medium are trappedwithin the said beam and neutralize the space charge therein, and meansadjacent said resonator for impressing upon said beam a magnetic fieldthe magnetic flux lines thereof being parallel to the paths of theelectrons of said beam.

10. Apparatus for producing varying microwave oscillations in a mediumincluding: means for projecting electrons along such paths that theytend to produce a focussed electron beam, means providing asubstantially field-free region through which said paths extend andincluding a gaseous medium, an electrode having a beamcurrent-limitingaperture adapted to limit the current of electrons which leave saidregion in accordance with the focus attained by said beam and anelectrode partially bounding said region for varying the fieldconfiguration thereof to control the focus attained by the beam bycontrolling ion entrapment in said region, a cavity resonator includingopposed walls each having an opening which surrounds the axis of saidbeam and is spaced from the other to define an interaction gap, electronreflector means positioned in said axis and on the side of said cavitystructure opposite to said beam producing means, whereby saidoscillations are produced in said cavity and whereby the positive ionsformed by the ionization of the medium are variably trapped Within thesaid beam and variably neutralize the space charge therein and saidoscillations are modulated in accordance with the said varyingconfiguration of said field, and means adjacent said resonator forimpressing upon said beam a magnetic field, the said magnetic flux linesthereof being parallel to the paths of the electrons of said beam.

11. Apparatus for producing oscillations in a gaseous medium including:means for producing a focussed electron beam in a substantiallyfieldfree region in said medium, a cavity resonator opposite walls ofwhich have aligned apertures in the axis of said beam, electronreflector means positioned in said axis and on the side of said cavityresonator opposite to said beam producing means, and magnetic meansadjacent said reflector means for impressing upon said beam a magneticfield, the said magnetic flux lines thereof being parallel to the pathsof the electrons of said beam, whereby oscillations are produced in saidcavity.

12. The method of producing oscillations comprising: providing a sourceof moving electrons in a medium and focussed into a beam, projectingsaid beam through a field-free region that includes a resonant cavity,bunching said electron beam, reflecting said bunched beam in the reversedirection into said cavity, whereby the positive ions formed by theionization of said medium are trapped within said beam and neutralizethe space charge therein, and simultaneously impressing upon said beamand said bunched beam a magnetic field, the said magnetic flux linesthereof being parallel to the paths of the electrons of said beam andsaid bunched beam.

ERNEST G. LINDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,289,952 Zowrykin July 14, 19422,320,860 Fremlin June 1, 1943 2,401,945 Linder June 11, 1946 2,403,795I-Iahn July 9, 1946 2,404,279 Dow July 16, 1946 2,460,332 Manifold"new"--- Feb. 1, 1949,

